Process for crosslinking cellulosic fibers during gas suspension of fibers



PROCESS R CROS LULOSIC FIBERS DUB GAS SUSPENSION OF FIBERS Filed Dec.15, 1965 WET PULP FLUFFER EXHAUST AIR 8 HEATERS 3s STEAMlELEC. ICOLLECTOR 28 3O 29 FIG. I

I I? I2 FIG. 2

United States Patent US. Cl. 162157 7 Claims ABSTRACT OF THE DISCLOSUREA method for producing crosslinked stiffened cellulosic fibers insubstantially individual fiber form. The method involves fiberimpregnation with the crosslinking agent followed by fluffing and dryingactions and then crosslinking in a matter of seconds by propelling thefibers through a curing zone of a gas at crosslinking temperature. Theprocess may involve screening of. the product to remove fiber nits andthe like. The process includes an aging step of the fiber in mass formprior to the fiuffing action, the aging in combination with the othersteps having the effect of providing a product substantially free ofnits and eliminating the necessity for screening.

This invention relates to the crosslinking of cellulosic fibers,particularly wood pulp fibers.

The impregnation of cellulose fibers with crosslinking agents followedby heating to occasion a crosslinking reaction is known. In general, itis considered that the reaction is that of an agent with the celluloseto crosslink adjacent cellulose chains. Commonly, this reaction betweenthe cellulose and the crosslinking agent is very limited in degree andrequires a significant period of time at elevated temperatures. Thetemperature to which the fibers may be subjected is limited because thefibers are, to some degree, heat sensitive and will char or burn if thefiber temperature becomes high. Therefore, many minutes of reaction timeare commonly involved in the crosslinking of wood pulp fiber with otheragents even at low degrees of crosslinking.

It is a purpose of this invention to provide a novel procedure for theattainment of crosslinked cellulosic fibers wherein the crosslinkedmaterial is attained within a short period of time which is at least anorder of magnitude different from the usual procedures. Also, it is apurpose of this invention to attain a degree of reaction sufiicient tosignificantly alter the characteristics of individual fibers and massesconstituted by such fibers. An allied purpose is to attain thecrosslinked fibers altered in form as to stiffness, surfacecharacteristics and the like while yet preventing significant fiberdamage, that is, the fiber is not materially shortened, abraded or thelike by the practice of the invention.

It is yet another object of the invention to provide a process for theattainment of crosslinked wood pulp fibers wherein the fibrous productof the reaction may require substantially no screening for removal offiber clumps and the like, although screening for improving uniformityof product may be practiced in one variation of the in vention.

The invention will be more fully understood by reference to thefollowing detailed description and accompanying drawings wherein:

FIG. 1 is a schematic representation in the general form of a flowdiagram illustrating a preferred embodiment of the invention; and

FIG. 2 illustrates a preferred arrangement of an equip- Patented Apr.22, 1969 ment component for the practice of one phase of the invention.

Similar parts are designated by corresponding numbers in the drawings.

In the practice of the preferred embodiment of the invention andreferring to FIG. I particularly, a roll of air dry pulp 1 is unreeledand directed in mat form to an aqueous solution of a crosslinkingimpregnant indicated at 3 and contained in any convenient type of tank 2utilizable for saturating the pulp. The pulp of roll 1 is suitablysouthern pine bleached kraft.

The pulp roll 1 for commercial application may have a very high basisweight, about 1000 pounds per 3000 square feet, although the practice ofthe invention is not restricted thereto. This pulp roll 1 will commonlyhave been produced in conventional pulp forming procedures and may beany conventional type pulp of a nature such that it could, with propertreatment, be formed into a usual self-sustaining paper sheet. Whetherof sulfate, sulfite, soda or a similar pulp, the pulp might also besupplied in laps, sheets and baled sheets as well as in roll form.Additionally, the pulp may be bleached or unbleached. The moisturecontent of the pulp at this stage suitably is air-dry, that is, aboutpulp and 10% moisture. The pulp may also contain additives such assurfactants for the purpose of aiding dispersion of the sheet; such arenot necessary to the practice of this invention but may be included andare present in some commercial pulps.

The pulp, in passing through the impregnating solution, moves as a sheetunder guide rolls 4, then between squeeze rolls 5 and is formed into asaturated pulp roll 6. The squeeze rolls 5 serve to press out excesssolution. The extent of saturation of the pulp may be controlled both bythe pressure applied by squeeze rolls 5 and the speed of movement of thepulp through the solution. This pulp sheet is commonly quite thick butwill saturate readily and the submergence time is usually only a fewseconds. Excessive pickup by the web may weaken and break the web and,accordingly, if so desired the web may be supported in its passagethrough the bath. Alternatively but less desirably, the impregnatingsolution may be applied by other modes of application including sprays.Preferably, the impregnant is present in the form of a saturatedsolution, that is, as high a concentration as possible.

The thick impregnated pulp mat will commonly have a solution pickupafter pressing of about 90% to 200% by weight and, therefore, a basisweight in this condition in the range of 1900 pounds to 3000 pounds whenthe air dry basis weight is 1000 pounds. The nature of the pulpdetermines to some extent the consistency after pressing; usually 40 to55% consistency is attained. High er consistencies are desirable tominimize drying costs but the pulp must remain wet upon pressing for mypurpose. Commonly, the bleached kraft will have a solution pickup ofabout 200%.

The impregnated wet pulp in subsequent steps is fiuffed and dispersed,dried, cured and collected. In the combination of steps the pulp is sodispersed as to provide a product which, in the optimum condition, is insubstantially individual fiber form. A mass of these cured fibersexhibit particularly desired characteristics, as noted hereinafter.

I have found that, to provide a fibrous product requiring little or noscreening, it is desirable to pre-treat the saturated pulp roll 6. Mypre-treatment preferably involves simply storing the wet impregnatedpulp for a period of time, about 16 to 48 hours. The storage conditionsmay be simply those of normal temperature and pressures, or theconditions may be controlled as to humidity, temperature and the like.Temperatures, for example, of 40 F. to 120 F. are suitable for thepurpose. With the kraft pulp specifically under consideration, a time of18 hours at 70 F. under atmospheric conditions is suitable. It isimportant that during the storage period, the pulp retain sufficientmoisture to inhibit migration as such would lead to areas of varyingconcentration of the impregnant particularly at roll edges.

In the absence of my pre-treatment step the subsequent fluffing action,as well as dispersion of the pulp during the drying and curing stages,are adversely affected. Thus, if the wet impregnated roll is feddirectly to the succeeding steps without pre-treatment, clumps of fiberstermed nits occur in the final product. For many purposes, particularlywhere absorbency characteristics of optimum quality are desired in thefinal product, the presence of such nits is essentially non-contributingto desired product properties. Also, the presence of nits tends tohinder drying of the material in the nits themselves and, While suchnits may be screened out of the cured product, this results in excessiveoperations and product loss, and, further, there then exists thepossibility of material of uncured nits being redispersed in thescreening operation and passing with the product, lowering its quality.

While I am unable to account for the effects imparted by pre-treatment,as in aging or the like, it has been found to be a most desirable stepwith respect to the efficiency of the subsequent mechanical handling ofthe pulp. Accordingly, I do not wish to be limited by any explanationset forth herein. It does appear that the impregnated pulp fibers,swollen by the impregnation, remain swollen, capable of further waterabsorption, and flexible but individually slightly stiffened by thepre-treatment. In any event, as indicated in FIG. 1, I prefer that thewet saturated pulp roll 6 be subjected to pre-treatment beforesubmission to a fiuffer or fiberizer and the pre-treatment should not besuch as to cause significant bonding between fibers of the rolledpressed sheet. The pre-treatment should stiffen the fibers somewhat andreduce their interbonding capacity.

The pre-treated, pressed pulp is unrolled and fed to fiberizer orfluffer 7 in the wet condition. Commonly, the pulp consistency at thisstage is 40 to 55%, substantially no water having been lost in thepre-treatment step. The fluffer 7 serves to separate the fibers to alarge degree into individual fibers. This action is carried out withoutsignificant cutting, shortening, tearing or mutilation of the fibers.Conventional equipment may be used for the purpose and such includeshammermills, disk refiners, impact mills and the like operating in knownmanner for fiberizing. Very little power input is required for thispurpose as compared with typical refining operations in which thecharacter of the pulp is changed by cutting, fibrillation and the like.Some such mill operations may act to break the pulp into small hardpieces, which is undesirable as difliculty may then be encountered inseparation into individual fibers in succeeding steps. Thus, undueimpact action which tends to form clumps should be avoided; suchfiberizing practice is and of itself known to the pulping art.

The output of the fiberizer 7 containing usually many individual fibersbut possibly some soft agglomerates is then fed to reaction chamber 8.Preferably, the feed is directly from the outlet of the fluffing deviceto the inlet of chamber 8, that is, one unit is coupled to the othermechanically by a length of conduit. Such is not necessary but ismechanically expedient. The pulp at this time is still well wetted.Thus, moisture loss in the fiuffer 7 may be minimal. However, if sodesired, drying may take place during flufling as long as the fibers arein the fluffed condition so that interfiber bonding does not take placeto a significant degree.

Reaction chamber 8 includes an educator 9 and a curing zone 10. Theeductor further includes a tapered air expansion zone or diffusing zone11.

While drying may take place in the fiberizer 7 and the eductor 9 is thennot necessary, I prefer to employ the eductor. Eductor 9 (FIG. 2) has astraight throat portion 12 welded at 13 to a plate 14 carried by aninternally bored cylinder 15. The bore 16 of cylinder 15 tapers from asmall diameter at the throat portion to an enlarged diameter in thedirection of the curing zone 10 (FIG. 1). The throat portion itself isprovided with a nut 17 having an aperture 18 in which there is pressfitted a small diameter tube 19 serving as a pulp inlet. A side aperture20 of the eductor provides for air inflow downstream of the pulp inlet.This structural arrangement provides for introduction of the wet pulpunder the influence of the inflow of air through the aperture 20 withonly a minimum of inflow of air from the atmosphere or from any devicecoupled to the reaction chamber such as a fiberizer.

The pulp is fed into tube 19 of eductor 9 and is impelled through theeductor by a hot air blast. Hot air under pressure is supplied from anyconvenient source designated generally at 22. Source 22 (FIG. 1)comprises an air compressor 23 from which air is directed successivelyover steam heaters 24 and electric heaters 25 to the eductor through theconduit indicated at 26. Alternatively, a furnace may be employed tosupply the heated air. The air (or gas from a furnace) directed to theeductor should be free of contaminants which might affect the desiredproperties of the final product adversely. Also, the heated air shouldbe at a sufficient temperature to effect rapid drying of the pulp; theair velocity should be high to attain a high drying rate and sufiicientto impel the pulp through air expansion zone 11 into the curing zone 10.Air velocity and temperature will vary somewhat with the fiber feed, theequipment size and the like; exemplary of conditions I have found usefulin the apparatus illustrated in the drawings and having parameters setforth hereinafter are an air temperature of 630 F. at a volume flow ofabout c.f.m. at about 2.5 p.s.i. gauge pressure. The high airtemperature does not injure the fiber due to the rapid evaporation ofthe water from the fiber, which maintains the fibers themselves suitablycool. The low pressure created within the eductor by the high flow rateof the combination of fiber and heated air aids rapid evaporation anddrying of the pulp. Evaporation occurs at a low temperature due to thelow pressure and maintains the fiber temperature well below 212 F. Theair passes with the dried fiber toward the curing zone 10 and ultimatelyfrom the equipment.

The fiber in the eductor at least initially has a straight line flow andthe fiber feed is at a high rate. Therefore, some fiber-to-fiber contactmost probably exists during drying and normally these fibers, if driedin contact, would be cured in contact and remain bonded. Such apparentlyoccurs to some extent in the absence of any pre-treatment and screeningof the cured product is then desirable. With pro-treatment the fibersare apparently slightly stiffened and are separated readily. Also, inthe eductor a turbulence is present which aids fiber separation and,further, flashing of water from the fibers may be a factor assistingseparation. However, if the nits are too large, neither turbulence orflashing is effective to completely demolish the nits.

The air expansion zone 11 serves to decrease air velocity gradually andto inhibit development of undesirable air motion such as eddy currents.

As the dry fiber emanates from the eductor to the curing zone, it isalready separated into substantially individual fibers. This permitspresentation of a very considerable fiber surface to the action of heatin the curing zone and apparently accounts for the ability to effectcuring and material change of fiber characteristics within a very shortspace of time.

For the purpose of effecting curing, I prefer to supply a second heatedair or gas stream to the curing chamber to attain or maintain reactiontemperature. This secondary air supply is lower in temperature generallythan the primary air to the eductor and must be sufficiently low toprevent burning or charring of the fibers as the fibers are nowsubstantially moisture-free. Secondary air is provided from a sourcesuch as a furnace or a source such as is generally designated at 27 inFIG. 1 comprising blower 28, electric heater 29, steam heater 30, andconduit 31. Conduit 31 directs the secondary air to the expansion zone11 at a point close to the fiber entrance to curing Zone 110; this pointof entry is not critical but the secondary air should be supplied at apoint after the eductor in the path of flow. Introduction of secondaryair into the eductor itself may cause unsatisfactory air flow reducingthe efficiency of operation. This secondary air not only provides heatfor the crosslinking reaction but maintains the fibers well suspended.An exemplary condition compatible with data already given is an airtemperature of about 430 F. at a volume flow of 250 c.f.m.

To assist in maintaining an optimum fiber suspension throughout theprocedure, the incoming air through conduits designated 26 and 31 may begiven a swirling or spiral motion. The provision of simple right anglebends as at 32 in conduit 26 serves the purpose of spiraling the primaryair flow; the tangential entry of air through conduit 31 complementsthis purpose. Though such spiral motion of the air (and the fiberstream) is not essential, it increases the relative velocity betweenfiber and the air and increases heat transfer to the dry impregnatedfibers, thereby aiding curing.

The residence time within the reaction chamber 8 for the air and fiberis estimated to be about 3 seconds. Such time may vary and is dependentupon a plurality of interrelated factors, including, to some extent, thetemperature conditions, the air-to-fiber ratio, the velocity of the airand fiber and physical characteristics of the reaction chamber such aslength and diameter. I have found that with a throat length of 3 /2 at adiameter of 1.15" and an overall eductor length including the diffusingor air expansion zone of about 20 and with temperature and air flow aspreviously stated, an overall length of chamber 8 of about 40 feetserves the purpose well. These chamber dimensions are simplyillustrative and may be varied considerably in specific applications.

An important facet of the curing action for optimum control is theintroduction of the secondary air and the maintenance of reactiontemperature with such air. The primary air cooled by its contact withthe fiber, the vaporization of the water from the fibers and educted airis apparently adequately raised in temperature by the hot secondary airstream so that rapid curing is accomplished. The secondary air might beeliminated so long as the primary air is sufficient in temperature inthe curing zone to effect reaction. This is dependent upon the initialprimary air temperature and the drop in air temperature due toevaporation of the water from the fiber. A lesser amount of waterevaporated will, of course, affect the air temperature to a lesserdegree.

I prefer, however, to introduce the secondary air as described. Suchserves to aid the pneumatic propulsion of the fibers in the highlyfluffed suspension and to provide for rapid increase in temperature ofthe dried fibers from below 212 F. to curing temperature withoutexcessive initial temperatures and volume of heating gas. The fibers inthe curing zone due to the substantially complete drying action arecollapsed about the retained crosslinking agent. It is believed quitesurprising both that the pneumatically propelled fibers may be obtainedtotally or very largely as individual fibers in the suspension and thatheat will penetrate the dry fiber wall sulficiently quickly to react thecrosslinking agent within a matter of a few seconds.

The pulp passing from the reaction chamber is directed to any suitablecollecting device which permits the removal of the air readily from thefiber. A cyclone designated at 34 serves the purpose of separating theair from the flowing stream, the air being exhausted at 35 and the fiberproduct being deposited through the bottom of the cyclone at 36 to anyconvenient collector or receptacle. Such may include bags, a screen orconveyor belt (not shown).

The fiber product is of excellent color, both G.E. brightness andGardner color measurements showing no significant difference between theoriginal pulp and the crosslinked fiber product. In the practice of theinvention the pulp fibers employed are of papermaking length andmicroscopic examination of the modified fibers indicates that the fiberlength distribution in the crosslinked material is substantially thesame as the fiber length distribution in the original untreated fibers.The fibers differ importantly from the original fibers in wet and drystiffness. Due to the crosslinking action while the fibers are dry andcollapsed and the set imparted by crosslinking while dry, the fibers areinhibited against swelling when wetted. Also, the fibers lack bondingcapacity for each other and are stiffened to the extent that mechanicalworking as in a refining operation in a papermaking process would reducethe fiber substantially to a powder.

The cured fiber before use in certain types of products such as sanitaryapplications may be subjected to washing with water to remove anypossible excess reactant material from the fibers. Such washing does notaffect the stiffness or the surface characteristics of the product.Additionally, the fibers may be redispersed in water for the purpose offorming mats of a low density 0.8 pound per cubic foot, for example) andhaving a high wet bulk, high Wet and dry resiliency, high porosity andabsorbency as well as a high wet stiffness. The individual fibersthemselves are quite rigid and substantially free of bonding capacitybetween themselves to the extent that they do not themselves form abonded paper sheet.

The crosslinked fibers, whether redispersed after their formation in themanner described or whether simply used directly without other working,when in mass form demonstrate materially improved water-holding capacitywith respect to a mass of the original fibers. This waterholdingcapacity is influenced by several processing factors and is a measure offiber stiffness. One such influencing factor I have found to be thepre-treatment of the wet saturated pulp fiber prior to its introductionto the flufier. Simply storing the saturated pulp at a water content ofabout 40 to 60% by weight for a period of time sufiicient to reduce theinterfiber bonding capacity results in a marked increase inwater-holding capacity of a mass of the fibers as well as the alreadynoted decrease in nit content. The improved water-holding capacity dueto my pre-treatment is apparently the result of the substantialelimination of nits and the procurement of individual fibers. Similarimproved water-holding capacity in the absence of pretreatment may beattained by screening of the product and elimianting larger clumpedmaterial.

The crosslinking impregnants are applied in solution form so that theimpregnant is within the fibers primarily rather than between fibers.The surface of individual fibers apparently necessarily is coated with avery thin film of the impregnant. Yet bonding between fibers, at leastirreversible bonding, does not take place during pretreatment even whena saturated solution of impregnant is applied to the fibers. Also, theprocess is apparently effective even with pulp in very heavy mat form,that is, mats ranging in basis weight from about pounds to 1000 poundsper 3000 square feet are useful and no significant bonding on agingoccurs.

The impregnants for crosslinking which I have found to be most usefulare combinations of formaldehyde and urea. The N-methylol ureas servethe purpose well. Dimethylol urea is available commercially andsuitable. Preferably, the aqueous impregnant solution is formed from theformaldehyde and urea components and the mole ratio of formaldehyde tourea should exceed about 1:1, a ratio of 1.5 :1 isuseful. This ratiowill be changed by the procedure since formaldehyde is commonly lost tothe atmosphere in the course of production.

An N-methylol urea solution found to be very useful for the practice ofthe invention is prepared by dissolving urea in water and then adding acommercially available composition designated as 25/60 urea-formaldehydewhich contains by weight 25% of urea, 60% of formaldehyde and 15% water.Such order of addition minimizes the quantity of fumes of formaldehydewhich escape. To the aqueous solution there is added sufficient alkalito bring the pH to about 8.5, and this alkaline solution is stored atleast overnight to permit the reaction between the urea and formaldehydeto take place to produce N-methylol urea. A typical formulation isprovided below:

1 Registered trademark of Rohm & Haas Company, Philadelphia, Pa

The surfactant such as Triton X-100 is not necessary but may aiddispersion of the fibers if the pulp is found to be dispersible withdifficulty. Too much of a surfactant is not desirable as it may affectthe surface properties of the product.

The concentration of the N-methylol urea in the solution specificallydescribed above is about 20% by weight and the mole ratio offormaldehyde to urea is about 2 /2 :1. This solution with ammoniumchloride added as catalyst to the extent of about l /2% by Weight basedon the N-methylol urea content by weight is the saturant solutionindicated at 3 in FIG. 1.

The crosslinking agent, the solution concentration of the agent, themole ratio of solution components and the catalyst and catalystconcentration may vary considerably while yet attaining a usefulproduct. Such crosslinking agents are themselves well known to the art.

I prefer to employ the N-methylol ureas as the crosslinking agent asthey are readily procured and relatively inexpensive. Solutionconcentrations of these compounds in the range of 12 to 30% are mostuseful. However, N- methylol compounds such as tri-methylol melamine andother similar polymerizable polyfunctional N-methylol compounds whichundergo condensation reactions are suitable particularly when theprocedure involves aging to decrease fiber bonding capacity. Dimethylolethylene urea, dimethoxy dimethyl uron and formaldehyde are furtherexamples of specific agents useful in the general practice of theinvention when aging is not involved. Formaldehyde, however, being morevolatile, is, for this reason, less desirable. Solution concentrationand impregnant pickup by weight, of course, may vary somewhat dependingupon the specific agent selected.

The catalyst may be any salt producing acid or acid commonly employed incrosslinking reactions. Such include mineral acids (HCl, H 80 Organicacids (acetic, formic) or acid salts (aluminum sulfate, magnesiumchloride, zinc chloride). While mineral acids have utility and are knownin the art as catalysts for the crosslinking reaction, they are notpreferred as they tend to attack the fiber. The organic acids and saltsare preferred. The catalyst should be selected in view of operatingconditions including the specific nature of available equipment(reaction chamber length, etc.). Too high a concentration of catalystmay lead to discoloration of the fiber, and too low a concentration maybe ineffective to fully develop the crosslinking reaction. I prefer withthe N-methylol ureas to employ ammonium chloride at a concentration ofabout 11% to 2% on the weight of N-methylol urea in the solution.

The time for complete drying and curing in my propulsion system has beenas low as about 1 and is always less than 10 seconds. In the specificembodiment described this time has commonly been about 3 seconds. Thisshort exposure time is not only advantageous production-wise but thefiber is less subject to thermal degradation.

With the N-methylol ureas I find that a mole ratio of formaldehyde tourea of between 2:1 to 3:1 in the initial solution to be most suitable.This ratio may be greater to the extent already noted that formaldehydealone is useful. These N-methylol urea compounds are complex andcommonly, for example, a solution designated as dimethylol urea maycontain proportions of other methylolurea. Accordingly, the exactoptimum ratio is not readily designated. Additionally, I have foundthat, if the wet pressed fiber sheet contains between about 18% to 3 8%by weight, based on the dry fiber weight, of N-methylol urea, thewater-holding capacity and consequently stiffness of the resultant curedfiber is not Widely different over the range. It is not necessary inorder to attain the product to employ such high loadings of reactant onthe fiber. As low as 7% of N-methylol urea in the wet pressed fibersheet (based on dry fiber weight) is effective to double thewater-holding capacity of a mass of the resultant fiber. However, higherloadings increase the water-holding capacity to as much as 3 times thatof the original fibers and, for production purposes, it is desirablethat the loading be to the high side. Higher concentrations of reactantin the fiber are also desirable as some material, particularlyformaldehyde, may be lost in processing. The loss will, of course, bedependent upon the specific system employed. I have found that as low acontent in the finally cured product as 10% of the crosslinking agentderived from dimethylol urea and exhibiting a formaldehyde content uponacid hydrolysis of about 3.9% by Weight is effective to more than doublethe water-holding capacity of the original fiber mass. The mole ratio insuch cured product of formaldehyde to urea is about 4:3.

In a further embodiment of the invention, commercially produced pulprolls have been treated to condition them for papermaking, that is, thepulp has been beaten to various Canadian Standard Freenesses and thensubjected to the impregnation, aging, drying and curing proceduredescribed hereinbefore. Specifically, bleached southern pine kraft pulphaving a Canadian Standard Freeness of about 380 and a water-holdingcapacity of about 7 grams of water per gram of fiber has been introducedinto the impregnating solution (FIG. 1), pressed, aged for about 18 to20 hours, fiberized and cross-linked as already described. The productwas as previously described.

When aging is practiced, screening is not necessary to attain a desiredproduct of optimum characteristics of absorbency, porosity and the likein mass fiber form. If screened, only 23% or lower of the product isrejected, and this quantity will vary somewhat depending on the specificnature of the ifibers, for example. Repeated tests have shown that anaging time of only six hours, for example, is effective to occasion areduction in the quantity of fiber rejected by screening. For example,aging for six hours in contrast to no aging period reduced the rejectweight by a factor of 3:1. As already noted, for many applications thepresence of some nits in the product may be tolerated, as in the pliesof paper toweling, for example, and no screening is then necessary.

Other papermaking fibers such as cotton linters, bast fibers, hemp andthe like may be beneficially employed in my process but I have found thegreater utility of the procedure, because of the cost factors and thelike, to be in the more conventional wood pulp fibers mentionedhereinbefore.

It will be understood that this invention is susceptible to modificationin order to adapt to different usages and conditions and, accordingly,it is desired to comprehend such modifications within the invention asmay fall within the scope of the appended claims.

What is claimed is:

1. A method of crosslinking cellulosic fibers to provide materiallystitfened fibers which includes the steps of saturating a mass ofcellulosic fibers of papermaking length with an aqueous catalyzedsolution of a polymerizable polyfunctional N-methylol compound havingcrosslinking capacity upon heating and which crosslinking agent insolution penetrates and is retained primarily within the cellulosicstructure of individual fibers of the mass, aging a wet mass of theimpregnated fibers for a sufiicient time to decrease the interfiberbonding capacity of the fibers, the fibers being sufiiciently wet duringaging to inhibit crosslinking agent migration,'then fiuffing in the wetstate the aged mass of saturated impregnated fibers to separate fibersof the mass while retaining crosslinking agent within the fibers,pneumatically propelling in gaseous suspension a stream of the flufiedwetted fibers and carrying out said pneumatic propulsion in a drying gasin a drying zone at sufiicient gas velocity and temperature so that saidfibers are dried, separated, collapsed about the retained crosslinkingagent in the fibers and are maintained below a temperature of 212 F.during drying and, without interrupting the propulsion of the'fibers ingaseous suspension, subjecting the dried collapsed separated fibers in acuring zone to gas at crosslinking temperature for a time to eifectcrosslinking of the agent retained within the cellulosic structure ofthe individual fibers whereby the individual fibers are stifiened,separating from the gas the dried stiffened fibers as substantiallyindividual fibers, and collect the stiffened fibers.

2. A method according to claim 1 wherein the cellulosic fibers are woodpulp fibers, the mass of fibers subjected to the saturation and fiutfingsteps is in the form of a pulp sheet having a basis weight of at leastabout 100 pounds per 3000 square feet on an air dry basis and the stepof aging is carried out for a period of at least about six hours atmoderate temperatures.

3. A method according to claim 1 wherein the cellulosic fibers are woodpulp fibers, the crosslinking agent is selected from the groupconsisting of N-methylol ureas and N-methylol melamines, and the fluffedwetted wood pulp is subjected to pneumatic propulsion through the dryingand curing zones for a period of time sufficient to dry the fibers andeffect crosslinking, said time being within from about one to less thanten seconds.

4. A method according to claim 1 wherein the temperature of the dryinggas to the drying zone exceeds the temperature of the gas of the curingzone when the latter is at crosslinking temperature.

5. A method according to claim 1 wherein the cellulosic fibers are woodpulp fibers, the crosslinking agent is an N-rnethylol urea compoundhaving a mole ratio of formaldehyde to urea of between about 2:1 to 3:1and the impregnated wood pulp fibers subjected to the gas atcrosslinking temperature contain between about 7 to 3 8% of thecrosslinking agent by weight based on the weight of the air dry fibers.

6. A method according to claim 1 wherein the cellulosic fibers are woodpulp fibers, the crosslinking agent is an N-rnethylol urea having aformaldehyde to urea ratio of between about 2:1 to 3:1, the wood pulpfibers are saturatedawith the aqueous catalyzed solution of thecrosslinking agent and are aged in the wet state at a water content ofabout 40 to by weight at a temperature of between about 40 F. and 120 F.prior to the fiuffing step-for a period of at least about six hours todecrease the interfiber bonding capacity of the fibers wherebyseparation of the wood pulp into individual fibers in the flulfing andpneumatic propulsion steps is facilitated, and wherein the pneumaticpropulsion step is carried out withafirst, a drying gas having atemperature in excess of the crosslinking temperature of thecrosslinking agent and, secondly, with a gas of a lower temperature butsufficient to effect rapid curing of the crosslinking agent.

7.A method according to claim 1 wherein the cellulosic fibers are woodpulp fibers, the crosslinking agent is an N-rnethylol urea having aformaldehyde to urea ratio of between about 2:1 to 3:1, the wood pulpfibers are saturated with the aqueous catalyzed solution of thecrosslinkingagent and are aged for a period of about 16-48 hours and,following aging, the fibers are subjected to the fluffing step at aWater content of about 40-55% by weight.

. References Cited UNITED STATES PATENTS 2,846,337 8/1958 Cooke et al.8-1163 X 3,055,795 9/1962 Eberhardt l62100 3,224,926 12/1965 Bernardin.3,316,141 4/1967 Bergholm etal 1s2 S. LEON BASHORE, Primary Examiner.

US. Cl. X.R.

8l16.3; 3410, 12; 162--l83; 264-1l6

